We have monitored the reflectivity of mirrors that were exposed to a f
luoroelastomer (3M-Fluorel 2176) and a room-temperature vulcanizing si
licone rubber (RTV-615) in vacuum. The 95% confidence limit on the dec
rease of mirror reflectivities was less than 0.35 ppm/week for Fluorel
and <0.29 ppm/week for RTV-615.